Mandarin not particularly useful in KL or Bkk. For first visit, definitely Bkk.....more to see, do and eat!

A trip to Siem Reap is also well worth 2-3 days, depending how long you want to spend at the various temples. Personally, I did a full tour day starting before sunrise with a guide/car and thought more than enough. Others will suggest more time. YMMV